1. Field of the Invention
The present invention relates to a turbo vacuum pump, and more particularly to an oil-free turbo vacuum pump which is capable of evacuating gas in a chamber from atmospheric pressure to high vacuum.
2. Description of the Related Art
Conventionally, in a semiconductor fabrication apparatus or the like, turbo vacuum pumps have been used for evacuating gas in a chamber to develop clean high vacuum (or ultra-high vacuum). These turbo vacuum pumps include a type of vacuum pump in which a turbo-molecular pump stage, a thread groove pump stage and a vortex pump stage are disposed in series in a pump casing having an intake port and a discharge port, and a main shaft to which rotor blades of these pump stages are fixed is supported by a hydrostatic gas bearing, a type of vacuum pump in which multiple pump stages are disposed in a pump casing having an intake port and a discharge port, and a main shaft to which rotor blades of the pump stages are fixed is supported by a hydrodynamic radial gas bearing, and a thrust bearing comprising a hydrodynamic thrust gas bearing and a permanent magnet, and other types of vacuum pumps. In this manner, the main shaft is supported by the gas bearing without using a rolling bearing to construct an oil-free turbo vacuum pump which does not require oil in the entirety of the pump including gas passages and bearing portions.
In the vacuum pump which is capable of compressing gas from ultra-high vacuum to atmospheric pressure, in order to reduce a flow rate of countercurrent flow of gas and improve compression capability, it is necessary to make blade clearance minute at a pressure close to atmospheric pressure. Further, when gas is compressed from ultra-high vacuum to atmospheric pressure, a thrust load acts on a rotor by the pressure differential. Thus, in the turbo vacuum pump, mechanical ball bearings are employed as a bearing for supporting the rotor having rotor blades of a pump stage. However, because physical contact occurs in the mechanical ball bearing, the rotor cannot be rotated at high speed. Besides the ball bearing, a gas bearing is used as a bearing for rotatably supporting the rotor with an accuracy of several micron meters (μm) to several tens of micron meters (μm). The turbo vacuum pump in which a rotor having rotor blades of a pump stage is supported by the gas bearing is disclosed in Japanese laid-open patent publication No. 2002-285987 or Japanese laid-open patent publication No. 6-193586, for example.
However, in order to generate a load equivalent to a thrust load caused by the pressure differential from the ultra-high vacuum to atmospheric pressure using the gas bearing disclosed in Japanese laid-open patent publication No. 2002-285987 or Japanese laid-open patent publication No. 6-193586, it is necessary to make the clearance of the gas bearing narrower, thus reaching the limit of part accuracy or the limit of machining and dimensional measurement. Specifically, it is difficult to manufacture a gas bearing having an extremely small clearance from the aspects of machining and measurement.
Further, in the case where a repulsive force of permanent magnet is utilized for canceling out the thrust load as disclosed in Japanese laid-open patent publication No. 6-193586, as clearance between the permanent magnet and an object facing the permanent magnet is narrower, the repulsive force of the permanent magnet becomes stronger. However, such repulsive force (i.e. clearance) cannot be controlled, and thus the thrust position of the rotor cannot be controlled.